An inkjet printing system typically comprises an image source and inkjet printhead. Such a system is designed to print a specific output on a substrate. The inkjet printhead typically further comprises a plurality of drop emitting devices, each drop emitting device being configured to emit a quantity of ink onto the substrate below. In a colour printing system a plurality of inkjet printheads are used, wherein each inkjet printhead is adapted to print a different colour component of the complete colour image.
Typically, an image source provides image data in the form of a two-dimensional image file. When an image is to be printed, print drivers upon the image source and control systems upon the inkjet printing device convert the image data into a series of control signals which are used to actuate the drop emitting devices. In a typical colour printing system, image data is first converted into four colour components: each component being one of cyan, magenta, yellow and black (CMYK). The printing system then comprises four sets of drop emitting devices, one on each of a plurality of inkjet printheads, wherein each device is adapted to print a different colour component. In most printing systems, each drop emitting device will be a binary device configured either to print a drop of ink upon receipt of a positive signal or not to print the drop of ink on receipt of a null signal. Hence the image data, which is typically within a colour space, such as RGB or CIE L*a*b* colour space, must be converted into a binary control signal with which to control each drop emitting device.
This conversion process is further complicated by the process of screening. As stated previously, an image is typically decomposed into four colour component images, one for each of the CMYK components. Each of these CMYK images is then printed by a separate set of drop emitting devices. By printing each component on top of each other, a colour image is produced. However, it is difficult to represent different shades of colour with this system, as the amount of ink emitted by each drop emitting device is limited and as such the printing process cannot vary the amount of ink applied to the substrate. Screening solves this problem by representing lighter shades as a series of dots or patterns rather than solid areas of ink. By configuring the screening pattern, a variety of shades can be produced which appear to the eye to form a continuous colour image.
A prior art system for applying a screening process is shown in FIG. 11. The screening operation is performed pixel by pixel, as each pixel of image data is typically printed by a single drop emitting device. The prior art binary screening system 1100 comprises a threshold array 1130. The threshold array 1130 is typically a two-dimensional array that contains data configured so as to generate a screening pattern on the substrate. The system 1100 receives a pixel 1110 of image data and also receives the pixel's X 1160 and Y 1170 position within an image to be printed. The X 1160 and Y 1170 positional information provides a set of indices with which to retrieve a threshold value T from the two-dimensional threshold array 1130. The system 1100 also extracts a colour tone value Cr from the image data. This colour tone value Cr is typically the intensity level of a particular colour component. The colour tone value Cr is then compared with the threshold value T using a binary comparator 1140. The output of the binary comparator 1140 is a control signal CS which is used to control the drop emitting device 1150.
For example, if the colour tone value Cr is greater than the threshold value T, the comparator 1140 will output a control signal CS with a value of one. This will instruct the drop emitting device 1150 to print a drop of ink. If the colour tone value Cr is less than the threshold value T, then the comparator 1140 will output a control signal CS with a value of zero which will instruct the drop emitting device 1150 not to print a drop of ink. By repeating this process for each pixel in a given scan line, and then repeating for the number of scan lines within the final image, a suitable screening pattern can be printed on the substrate. As the X 1160 and Y 1170 positional data for each pixel will vary, a number of different threshold values will be generated. It is this variation that allows a screening pattern to be printed.
The method of printing an output using the system of FIG. 11 is illustrated in FIG. 1. At step 110 image data Ir is received. This image data Ir is converted to a control signal value CS by the system of FIG. 11 in step 120. The control signal is then sent to a drop emitting device in step 130 and the drop emitting device prints an output onto the substrate at step 140. At step 150, the printing system 1100 checks whether more image data remains: if there are remaining pixels then the printing system 1100 retrieves the next pixel at step 160 and the process begins again. If there are no more remaining pixels then the process ends at step 170.
In recent times, inkjet printing devices have been invented that can emit a plurality of ink quantities. For example, these inkjet printing devices may comprise adapted drop emitting devices that are able to emit two or more drops for a given pixel or pixel colour component value. These devices enable printed images with greater shade fidelity to be generated. However, the use of these devices also generates two additional problems. The first of these is how to adapt the binary screening system such as that shown in FIG. 11 which is designed for binary drop emitting devices. The second of these is how to reduce the appearance of undesirable print artefacts that have been created because a range of ink quantities can now be printed. These undesirable print artefacts arise due to the larger range of the adapted drop emitting devices, which means it becomes possible to print a reduced quantity of ink upon the substrate when compared to a binary drop emitting devices. This then becomes a problem when the resolution of the printing system in a direction perpendicular to the feed direction of a substrate is fixed by the number of drop emitting devices that form the inkjet printhead.
For example, an inkjet printhead may comprise a row of drop emitting devices that extends laterally in a direction perpendicular to the feed direction of a substrate. Using the latest technology this row may contain approximately 140 drop emitting devices per centimeter, fixing the resolution at 360 dots per square inch. At this resolution a binary drop emitting device is able to emit enough ink to cover a corresponding pixel area on the substrate below. However, if adapted drop emitting devices are used that can emit a reduced quantity of ink, then it is possible that the emitted ink will not cover the corresponding pixel area on the substrate below. In this case a viewer of the printed image may be aware of an area of unprinted substrate around the reduced quantity of ink present on the substrate. If two neighbouring adapted drop emitting devices emit a reduced quantity of ink then the problem is further compounded as this action produces an even larger area of unprinted substrate and can produce a “speckling” or “streaking” effect visible to a viewer. One such example is what is referred to as “rain”. This is where a series of vertical lines or streaks are visible on a printed image because a group of neighbouring adapted drop emitting devices have repeatedly been instructed to print reduced ink quantities.